1. Field of the Invention
The present invention relates to a sheet post-treatment apparatus which facilitate jam treatment, and more particularly, it relates to a sheet post-treatment apparatus for post-treating output copies discharged from an image forming apparatus such as a copying machine, a laser beam printer, a printing device and the like and successively sorted in a sheet receiving trays (referred to as "bins" hereinafter), and an image forming apparatus having such a sheet post-treatment apparatus.
2. Related Background Art
Conventionally, a sorter which is one of sheet post-treatment apparatuses serves to sort sheets discharged from an image forming apparatus in bins which are arranged side by side in a vertical direction or a horizontal direction, thereby obtaining a plurality of parts of copy bundles. In general, such a sorter comprises a sheet convey portion for conveying a sheet discharged from the image forming apparatus to the bins, and a bin portion including a plurality of bins on which the sheets are to be stacked.
In one of sheet sorting methods, the bins are selectively shifted to a discharge opening of the sheet convey portion (i.e. a sorter of so-called bin shift type). The bin portion and the sheet convey portion are fixedly arranged in proximity to each other so that they are not contacted with each other and are not separated from each other.
In the sorter of bin shift type, a position of the sheet conveyed in the sheet convey portion is always checked by a sensor or sensors. Further, the presence/absence of the sheet on the bin can be detected by a sensor attached to the bin portion. However, there was no means for ascertaining the fact that the sheet is correctly transferred from the convey portion to the bin portion. Thus, the conventional sorters had the following drawbacks.
If abnormal sheet convey occurs (for example, if the sheet is greatly curved or bent or if the sheet is skew-fed with respect to a sheet conveying direction during the conveyance of the sheet), the sheet discharged from the sheet convey portion will not be correctly rested on the bin so that, as shown in FIG. 8, the sheet is straddling between discharge rollers 501 of the sheet convey portion and the bin portion 502. In this condition, if the bin portion is shifted upwardly or downwardly, the sheet will enter into a clearance between the bin portion and the sheet convey portion, thereby causing the inconvenience (for example, the discharge opening of the sheet convey portion is closed by the sheet)(FIGS. 9 and 10).
If the discharge opening 502 is immediately closed by the sheet entered into the clearance, since a next sheet cannot be discharged through the discharge opening, the fact that the next sheet cannot be discharged is detected by a sensor (not shown) disposed in a sheet convey path of the convey portion, thereby informing the sorter of the abnormity. Consequently, the sorter and the image forming apparatus sending the sheet to the sorter are stopped, and an operator is informed of the fact that the sheet closing the discharge opening 502 should be removed. Since a distance between the bins in the proximity of the discharge opening 502 is wider than a distance between the bins in the other positions, the operator can relatively easily insert his hand between the bins and remove the sheet jammed in the proximity of the discharge opening. However, this sheet removing operation is troublesome in comparison with the normal jam treatment (in which a sheet jammed in the convey portion due to the poor sheet conveyance is removed), particularly for any operator having large hands.
Further, if the discharge opening of the convey portion is not immediately closed by the sheet S entered into the clearance between the convey portion and the bin portion, the sheet will be shifted to a position where the sheet cannot be easily removed (position S' shown in FIG. 9). That is to say, the sheet S straddling between the bin portion and the convey portion is shifted by the upward or downward movement of the bin portion or by being urged by the next sheet subsequently entering between the convey portion and the bin portion, with the result that the sheet is shifted to a position where the operator cannot easily access to the sheet. However, since the apparatus cannot ascertain the fact that the sheet exists in the abnormal position, the apparatus continues to operate. As a result, even if the fact that the sheet has entered between the bin portion and the convey portion is detected by the obstruction of the shifting movement of the bin portion due to the sheet jam and the sorter and the image forming apparatus sending the sheet to the sorter are stopped, it is impossible for the operator to remove the sheet S' thereby to restore the apparatus to the usable condition.